All patients with refractory and relapse lymphoblastic leukemia will receive a high-dose chemotherapy regimen consisting of fludaraibine+cytarabine+ cyclophosphamide+etoposide. At day 7 after chemo, a bone marrow aspiration and MRD study by flow-cytometry is planned. For those patients with no residual blasts will undergo allogeneic stem cell transplantation with reduced intensity condiotning regimen consisting of 5-day fludarabine and 3-day busulfan and post-infusion cyclophophamide (D+3 and +4) as GVHD prophylaxis.